In all rotating machines, there are some sort of radial bearings between the rotor part and the stator part. Most radial bearing types are arranged for supporting radial forces between a rotor and a stator by mechanical contacts. Typical examples of bearing types used as radial bearings are ball bearings, roller bearings and sliding bearings.
All these bearings are based on some kind of indirect mechanical contacts between the rotary and stationary part, play is an important factor. In an ideal case, a bearing would be play free, only permitting e.g. a thin film of lubricant between the moving parts. However, in typical cases a small play is present between the stator and the bearing and/or between the bearing and the rotor.
When rotating machines are operating, heat is produced, e.g. in the bearings. The heat causes the bearings to expand. If the expansion becomes larger than the available play, the rotating machine will be severely damaged. In order to allow operation at different temperatures, an extended radial play is typically provided for the bearings. The play has furthermore to be so large that it can handle a worst case scenario, i.e. the highest temperature at which operation of the rotating machine can take place. This results in that a rotating machine normally operates with a play that is somewhat larger than the optimum play.
The size of the play has impacts on the properties of the rotating machine. A large play will typically increase wear, and give a higher level of vibrations and noise in the bearing and its connected parts. Moreover, a rotor carried by a bearing will also experience different stiffness depending on the play. Generally, a small play is to prefer. At substantially no plays, resonance frequencies for the rotor can be well estimated and thereby avoided. At small plays, the vibration behaviour is somewhat different and difficult to calculate in advance. At very large plays, torques of the rotor and external forces acting on the rotor may cause the rotor to enter into a more or less chaotic condition.
The impact of the bearing plays becomes more important for large machines. In particular, large machines, having a large mass present at a large diameter of the rotor, are subjects for great concern. One typical such rotating machine is a refiner, used for refining of fibre materials, where grinding plates with substantial radial extension are rotated around an axis.
There are approaches in prior art to compensate for temperature induced play changes. In U.S. Pat. Nos. 3,418,809, 3,459,460 and 4,626,111, bearing arrangements with rollers arranged at tapered surfaces are disclosed, which by means of hydrostatic pressure, electrical heated expansion components or heat expanding bars causes a movement of the rollers. This results in a changed play. Such arrangements have the disadvantage of including additional features around and within the bearing and thereby increasing the risk for damages during operation.
In U.S. Pat. No. 6,261,003, an apparatus for controlling radial play of a roller bearing is disclosed. A lubricant drainage circuit outside a roller bearing is fitted such that an output drainage through a drained ring is controlled by providing drainage channels crossing the ring and having different inclinations in a circumferential direction of the drained ring. The lubricant is thereby utilized to reduce the temperature of the outer ring, giving a controlled play. The temperature of the lubricant is in turn determined by friction generated in the bearing. There are, however, a number of disadvantages with such a solution. The temperature control is connected to the flow and temperature of the lubricant. A flow and temperature advantageous for the temperature control aspect may not always be advantageous also for the lubricating purposes. Furthermore, the abilities to control the temperature are limited by the maximum lubricant flow and the temperatures at other parts of the bearing, in turn being dependent e.g. on load and friction. Moreover, the proposed approach can only control the temperature in one direction, typically a cooling of the outer ring and thereby increasing the play.
When starting a cold rotating machine that normally operates at a certain elevated temperature, the play is typically larger than at continuous operation. A play reduction during the starting-up phase can not be achieved by the approach presented in U.S. Pat. No. 6,261,003. There is thus a remaining risk for damaging operation, e.g. due to uncontrolled vibration states, before any steady-state operation is reached. Furthermore, the solution requires additional openings in the support surfaces of the bearing, which reduces the mechanical strength and increases the wear.
Prior art solutions of bearing play compensation have different disadvantages. A general problem is, however, the introduction of weakening or disturbing features within the bearing.
One object of the present invention is to provide a bearing system, which improves the possibilities to control the bearing play without substantially influencing the operation of the bearing itself. A further object is to provide a bearing play control system being able to both increase and decrease the play. Yet an object is to allow for an improved control of vibration behaviours of rotating machines.